Pressure Effect on Molecular and Lattice Dynamics in Pyridinium Nitrate

Abstract

Measurements of spin-lattice relaxation time T 1 , second moment M 2 and neutron scattering have been performed for a polycrystalline sample of pyridinium nitrate as a function of hydrostatic pressure and temperature. The structure of this compound has also been analysed by the HF/6-31 G method. The results of the measurements and calculations have confirmed that reorientation of the pyridinium cation takes place between the potential minima of different values, and the asymmetry parameter decreases with increasing temperature. The temperature dependence of the asymmetry parameter is modified by the pressure applied. The presence of the hydrogen bond in this compound implies a very small activation volume and is responsible that the crystal does not undergo a phase transition to the disordered phase.

Dynamics in molecular and molecular-ionic crystals: A combined experimental and molecular simulation study of reorientational motions in benzene, pyridinium iodide, and pyridinium nitrate

“…Therefore, similarly as for the low temperature phase of (PyH)I, also in (PyH)NO 3 the pyridinium cation reorientations take place in an asymmetric potential that undergoes changes with temperature. As follows from the MD simulations and from earlier NMR measurements as a function of temperature and pressure, 3,4 with increasing temperature the population of the potential energy minima is gradually equalized. Moreover, starting from 200 K, at which the populations of the two minima of potential energy begin to equalize, the hydrogen bond N-H· · · O length begins to increase and the N-H· · · O angle begins to decrease.…”

“…The planar pyridinium cation has a dipole moment and the possible anions may have different shapes: spherical in pyridinium bromide 1 and pyridinium iodide (hereafter (PyH)I), 1, 2 planar in pyridinium nitrate ((PyH)NO 3 ), 3,4 tetrahedral in pyridinium tetrafluoroborate, 1, 5, 6 pyridinium perchlorate, 7,8 pyridinium periodate, 9 and pyridinium perherate 10 , and octahedral in pyridinium hexafluorophosphate 11,12 and pyridinium hexafluoroantimonate. 13 All these salts except pyridinium nitrate present at least one phase transition.…”

“…Much effort has been made to prove that the pyridinium cation really reorients in a temperature dependent asymmetric potential, as such potential allows describing the ferroelectric properties of these compounds. Thus, the compounds pyridinium iodide, 2, 14-16 pyridinium tetrafluoroborate, 17 pyridinium perchlorate, 18,19 pyridinium periodate, [20][21][22] pyridinium perherate, 23,24 and pyridinium nitrate 4 have been studied by 1 H nuclear magnetic resonance (NMR) and inelastic neutron scattering as a function of temperature and pressure and by 2 H NMR and quasielastic neutron scattering (QENS) as a function of temperature. A detailed analysis of the dynamics of molecules or ions provides important information to understand their properties or the mechanism of their phase transitions.…”

The p–T phase diagram for ferroelectric bis-thiourea pyridinium nitrate

“…The parameters Da, V m , DV m , DS, DC p appearing in equations (1) and (2) were determined experimentally at the atmospheric pressure, so dT 1 /dp and dT 2 /dp were estimated for the pressure close to zero. The parameters needed for determination of dT 1 /dp and dT 2 /dp: Da, V m and DV m were obtained from measurements of the lattice constants as a function of temperature [6] (figure 5), while DS and DC p were obtained from calorimetric DSC measurements [9]. These values are DV m ffi ð0:9 AE 0:5Þ10 À6 m 3 , DS ffi ð12 AE 1Þ J Á K Á mol À1 , Da ffi ð0:64 AE 0:07Þ10 À4 K À1 , V m = (207.3 ± 0.2)10 À6 m 3 , whereas dT 1 /dp = 7 ± 2 K/100 MPa and dT 2 / dp = 14±2 K/100 MPa.…”

“…NMR and IINS studies performed as a function of temperature and pressure (up to 0.8 GPa) have revealed the modification of the energetic barriers for reorientation and permitted determination of the corresponding activation volume [6]. Neutron diffraction study of this compound as a function of pressure (up to 3.5 GPa) has shown that at 1 GPa the compound undergoes an isostructural phase transition [7].…”